Power transmission devices having torque transfer coupling with power-operated clutch actuator

ABSTRACT

A torque transfer mechanism is provided for controlling the magnitude of a clutch engagement force exerted on a multi-plate clutch assembly that is operably disposed between a first rotary and a second rotary member. The torque transfer mechanism includes a power-operated face gear clutch actuator for generating and applying a clutch engagement force on the clutch assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/287,708 filed Nov. 28, 2005 which claims the benefit of U.S.Provisional Application No. 60/647,331, filed on Jan. 26, 2005, theentire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to power transfer systems forcontrolling the distribution of drive torque between the front and reardrivelines of a four-wheel drive vehicle and/or the left and right wheelof an axle assembly. More particularly, the present invention isdirected to a power transmission device for use in motor vehicledriveline applications having a torque transfer mechanism equipped witha power-operated clutch actuator that is operable for controllingactuation of a multi-plate friction clutch assembly.

BACKGROUND OF THE INVENTION

In view of increased demand for four-wheel drive vehicles, a plethora ofpower transfer systems are currently being developed for incorporationinto vehicular driveline applications for transferring drive torque tothe wheels. In many vehicles, a power transmission device is operablyinstalled between the primary and secondary drivelines. Such powertransmission devices are typically equipped with a torque transfermechanism which is operable for selectively and/or automaticallytransferring drive torque from the primary driveline to the secondarydriveline to establish a four-wheel drive mode of operation.

A modern trend in four-wheel drive motor vehicles is to equip the powertransmission device with a transfer clutch and anelectronically-controlled traction control system. The transfer clutchis operable for automatically directing drive torque to the secondarywheels, without any input or action on the part of the vehicle operator,when traction is lost at the primary wheels for establishing an“on-demand” four-wheel drive mode. Typically, the transfer clutchincludes a multi-plate clutch assembly that is installed between theprimary and secondary drivelines and a clutch actuator for generating aclutch engagement force that is applied to the clutch plate assembly.The clutch actuator typically includes a power-operated device that isactuated in response to electric control signals sent from an electroniccontroller unit (ECU). Variable control of the electric control signalis frequently based on changes in the current operating characteristicsof the vehicle (i.e., vehicle speed, interaxle speed difference,acceleration, steering angle, etc.) as detected by various sensors.Thus, such “on-demand” power transmission devices can utilize adaptivecontrol schemes for automatically controlling torque distribution duringall types of driving and road conditions.

A large number of on-demand power transmission have been developed whichutilize an electrically-controlled clutch actuator for regulating theamount of drive torque transferred through the clutch assembly to thesecondary driveline as a function of the value of the electrical controlsignal applied thereto. In some applications, the transfer clutchemploys an electromagnetic clutch as the power-operated clutch actuator.For example, U.S. Pat. No. 5,407,024 discloses a electromagnetic coilthat is incrementally activated to control movement of a ball-ramp driveassembly for applying a clutch engagement force on the multi-plateclutch assembly. Likewise, Japanese Laid-open Patent Application No.62-18117 discloses a transfer clutch equipped with an electromagneticclutch actuator for directly controlling actuation of the multi-plateclutch pack assembly.

As an alternative, the transfer clutch may employ an electric motor anda drive assembly as the power-operated clutch actuator. For example,U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having atransfer clutch equipped with an electric motor that controls rotationof a sector plate which, in turn, controls pivotal movement of a leverarm for applying the clutch engagement force to the multi-plate clutchassembly. Moreover, Japanese Laid-open Patent Application No. 63-66927discloses a transfer clutch which uses an electric motor to rotate onecam plate of a ball-ramp operator for engaging the multi-plate clutchassembly. Finally, U.S. Pat. Nos. 4,895,236 and 5,423,235 respectivelydisclose a transfer case equipped with a transfer clutch having anelectric motor driving a reduction gearset for controlling movement of aball screw operator and a ball-ramp operator which, in turn, apply theclutch engagement force to the clutch pack.

While many on-demand clutch control systems similar to those describedabove are currently used in four-wheel drive vehicles, a need exists toadvance the technology and address recognized system limitations. Forexample, the size and weight of the friction clutch components and theelectrical power and actuation time requirements for the clutch actuatorthat are needed to provide the large clutch engagement loads may makesuch system cost prohibitive in some motor vehicle applications. In aneffort to address these concerns, new technologies are being consideredfor use in power-operated clutch actuator applications.

SUMMARY OF THE INVENTION

Thus, its is an object of the present invention to provide a powertransmission device for use in a motor vehicle having a torque transfermechanism equipped with a power-operated clutch actuator that isoperable to control engagement of a multi-plate clutch assembly.

As a related object, the torque transfer mechanism of the presentinvention is well-suited for use in motor vehicle driveline applicationsto control the transfer of drive torque between a first rotary memberand a second rotary member.

According to one preferred embodiment, a transfer unit is provided foruse in a four-wheel drive motor vehicle having a powertrain and firstand second drivelines. The transfer unit includes a first shaft drivenby the powertrain, a second shaft adapted for connection to the seconddriveline and a torque transfer mechanism, The torque transfer mechanismincludes a friction clutch assembly operably disposed between the firstshaft and the second shaft and a clutch actuator assembly for generatingand applying a clutch engagement force to the friction clutch assembly.The clutch actuator assembly includes an electric motor, a geared driveunit and a clutch apply operator. The electric motor drives the geareddrive unit which, in turn, controls the direction and amount of relativerotation between a pair of cam members associated with the clutch applyoperator. The cam members support rollers which ride against tapered orramped cam surfaces. The contour of the ramped cam surfaces cause one ofthe cam members to move axially for causing corresponding translation ofa thrust member. The thrust member applies the thrust force generated bythe cam members as a clutch engagement force that is exerted on thefriction clutch assembly. A control system including vehicle sensors anda controller are provided to control actuation of the electric motor.

In accordance with the present invention, the transfer unit isconfigured as an in-line torque coupling for use in adaptivelycontrolling the transfer of drive torque from the powertrain to the reardrive axle of an all-wheel drive vehicle. Pursuant to relatedembodiments, the transfer unit is a transfer case for use in adaptivelycontrolling the transfer of drive torque to the front driveline in anon-demand four-wheel drive vehicle or between the front and reardrivelines in a full-time four-wheel drive vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention willbecome apparent to those skilled in the art from analysis of thefollowing written description, the appended claims, and accompanyingdrawings in which:

FIG. 1 illustrates the drivetrain of an all-wheel drive motor vehicleequipped with a power transmission device of the present invention;

FIG. 2 is a schematic illustration of the power transmission deviceshown in FIG. 1 associated with a drive axle assembly;

FIG. 3 is a sectional view of the power transmission device which isequipped with a torque transfer mechanism according to the presentinvention;

FIG. 4 is an enlarged partial view taken from FIG. 3;

FIGS. 5-8 are schematic illustrations of alternative embodiments for thepower transmission device of the present invention;

FIG. 9 illustrates the drivetrain of a four-wheel drive vehicle equippedwith another version of the power transmission device of the presentinvention;

FIGS. 10 and 11 are schematic illustrations of transfer cases adaptedfor use with the drivetrain shown in FIG. 9; and

FIG. 12 is a schematic view of a power transmission device equipped witha torque vectoring distribution mechanism according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a torque transfer mechanism thatcan be adaptively controlled for modulating the torque transferredbetween a first rotary member and a second rotary member. The torquetransfer mechanism finds particular application in power transmissiondevices for use in motor vehicle drivelines such as, for example, anon-demand transfer clutch in a transfer case or an in-line torquecoupling or a biasing clutch associated with a differential unit in atransfer case or a drive axle assembly. Thus, while the presentinvention is hereinafter described in association with particulararrangements for use in specific driveline applications, it will beunderstood that the arrangements shown and described are merely intendedto illustrate embodiments of the present invention.

With particular reference to FIG. 1 of the drawings, a drivetrain 10 foran all-wheel drive vehicle is shown. Drivetrain 10 includes a primarydriveline 12, a secondary driveline 14, and a powertrain 16 fordelivering rotary tractive power (i.e., drive torque) to the drivelines.In the particular arrangement shown, primary driveline 12 is the frontdriveline while secondary driveline 14 is the rear driveline. Powertrain16 is shown to include an engine 18 and a multi-speed transmission 20.Front driveline 12 includes a front differential 22 driven by powertrain16 for transmitting drive torque to a pair of front wheels 24L and 24Rthrough a pair of front axleshafts 26L and 26R, respectively. Reardriveline 14 includes a power transfer unit 28 driven by powertrain 16or differential 22, a propshaft 30 driven by power transfer unit 28, arear axle assembly 32 and a power transmission device 34 for selectivelytransferring drive torque from propshaft 30 to rear axle assembly 32.Rear axle assembly 32 is shown to include a rear differential 34, a pairof rear wheels 36L and 36R and a pair of rear axleshafts 38L and 38Rthat interconnect rear differential 34 to corresponding rear wheels 36Land 36R.

With continued reference to the drawings, drivetrain 10 is shown tofurther include an electronically-controlled power transfer system forpermitting a vehicle operator to select a locked (“part-time”)four-wheel drive mode, and an adaptive (“on-demand”) four-wheel drivemode. In this regard, power transmission device 34 is equipped with atransfer clutch 50 that can be selectively actuated for transferringdrive torque from propshaft 30 to rear axle assembly 32 for establishingthe part-time and on-demand four-wheel drive modes. The power transfersystem further includes a power-operated clutch actuator 52 foractuating transfer clutch 50, vehicle sensors 54 for detecting certaindynamic and operational characteristics of motor vehicle 10, a modeselect mechanism 56 for permitting the vehicle operator to select one ofthe available drive modes, and a controller 58 for controlling actuationof clutch actuator 52 in response to input signals from vehicle sensors54 and mode selector 56.

Power transmission device, hereinafter referred to as torque coupling34, is shown schematically in FIG. 2 to be operably disposed betweenpropshaft 30 and a pinion shaft 60. As seen, pinion shaft 60 includes apinion gear 62 that is meshed with a hypoid ring gear 64 that is fixedto a differential case 66 of rear differential 34. Differential 34 isconventional in that pinions 68 driven by case 66 are arranged to driveside gears 70L and 70R which are fixed for rotation with correspondingaxleshafts 38L and 38R. Torque coupling 34 is shown to include transferclutch 50 and clutch actuator 52 arranged to control the transfer ofdrive torque from propshaft 30 to pinion shaft 60 and which togetherdefine the torque transfer mechanism of the present invention.

Referring primarily to FIGS. 3 and 4, the components and function oftorque coupling 34 will be disclosed in detail. As seen, torque coupling34 generally includes a housing 72, an input shaft 74 rotatablysupported in housing 72 via a bearing assembly 76, transfer clutch 50and clutch actuator 52. A yoke 78 is fixed to a first end of input shaft74 to permit connection with propshaft 30. Transfer clutch 50 includes ahub 80 fixed for rotation with input shaft 74, a drum 82 fixed forrotation with pinion shaft 60, and a multi-plate clutch pack 84comprised of alternating inner and outer clutch plates that are disposedbetween hub 80 and drum 82. As shown, a bearing assembly 86 rotatablysupports a second end of input shaft 74 in drum 82 which, in turn, isrotatably supported in housing 72 via a bearing assembly 88.

Clutch actuator assembly 52 is generally shown to include an electricmotor 90, a geared drive unit 92 and a clutch apply operator 94.Electric motor 90 is secured to housing 72 and includes a rotary outputshaft 96 having a drive gear 98. Geared drive unit 92 generally includesa first drive component 100 and a second drive component 102 that aresupported for rotation relative to each other and input shaft 74. Inparticular, first drive component 100 includes a cylindrical first hubsegment 104 rotatably supported via a bearing assembly 106 on inputshaft 74 and a radially extending first ring segment 108 located at afirst end of first hub segment 104. Second drive component 102 includesa cylindrical second hub segment 110 rotatably supported via a bearingassembly 112 on hub segment 104 and a radially extending second ringsegment 114 located at a first end of second hub segment 110. A thrustplate 116 and a lock ring 118 are provided to axially position geareddrive unit 92 on input shaft 74. As seen, first ring segment 108includes first gear teeth 120 on its inner face surface 122 that aremeshed with drive gear 98. Likewise, second ring segment 114 includessecond gear teeth 124 on its outer face surface 126 that are also meshedwith drive gear 98. Thus, rotation of drive gear 98 in a first directionresults in a corresponding amount of relative rotation between firstdrive component 100 and second drive component 102 in a first direction.Furthermore, rotation of drive gear 98 in the opposite second directionresults in a corresponding amount of relative rotation between first andsecond drive components 100 and 102 in a second direction. As will bedetailed, this bi-directional control over the rotation of first drivecomponent 100 relative to second drive component 102 is utilized tocontrol accurate and quick engagement of clutch pack 84 through clutchapply operator 94.

Clutch apply operator 94 is best shown in FIG. 4 to include a first camplate 130 fixed via a spline connection 132 to a second end of first hubsegment 104, a second cam plate 134 fixed via a spline connection 136 toa second end of second hub segment 110, and rollers 138. A roller 138 isdisposed in each of a plurality of aligned cam grooves 140 and 142formed in corresponding facing surfaces of first and second cam plates130 and 134, respectively. Preferably, three equally-spaced sets of suchfacing cam grooves 140 and 142 are formed in cam plates 130 and 134,respectively. Grooves 140 and 142 are formed to include cam surfaces140A and 142A, respectively, that are ramped, tapered or otherwisecontoured in a circumferential direction. Rollers 138 roll against camsurfaces 140A and 142A so as to cause axial movement of first cam plate130 relative to second cam plate 134 in response to relative rotationtherebetween. As seen, a first thrust bearing assembly 144 is disposedbetween first cam plate 130 and clutch pack 84 while a second thrustbearing assembly 146 is disposed between second cam plate 134 and aretainer plate 148 that is axially located on drum 88 via a lock ring150. Another lock ring 152 is provided to axially restrain second camplate 134 on second hub segment 110 of second drive component 102.

First cam plate 130 is axially moveable relative to clutch pack 84between a first or “released” position and a second or “locked”position. With first cam plate 130 in its released position, a minimumclutch engagement force is exerted on clutch pack 84 such that virtuallyno drive torque is transferred from input shaft 74 through clutch pack84 to pinion shaft 60 so as to establish a two-wheel drive mode. Incontrast, location of first cam plate 130 in its locked position causesa maximum clutch engagement force to be applied to clutch pack 84 suchthat pinion shaft 60 is, in effect, coupled for common rotation withinput shaft 74 so as to establish a “locked or part-time” four-wheeldrive mode. Therefore, accurate bi-directional control of the axialposition of first cam plate 130 between its released and lockedpositions permits adaptive regulation of the amount of drive torquetransferred from input shaft 74 to pinion shaft 60, thereby establishingthe on-demand four-wheel drive mode.

The tapered contour of cam surfaces 140A and 142A is selected to controlthe axial translation of first cam plate 130 relative to clutch pack 84from its released position to its locked position in response to drivegear 98 being driven by motor 90 in a first rotary direction. Suchrotation of drive gear 98 in a first direction causes first drivecomponent 100 to be angularly driven in a direction opposite to that ofsecond drive component 102, thereby causing a corresponding amount ofrelative rotation between cam plates 130 and 134 such that rollers 138ride against contoured cam surfaces 140A and 142A. However, since secondcam plate 134 is restrained against axial movement, the relativerotation causes axial movement of first cam plate 130 to its lockedposition for applying the maximum clutch engagement force on clutch pack84. Likewise, first cam plate 130 is axially translated from its lockedposition back to its released position in response to drive gear 98being rotated in a second direction through the same amount of angulartravel. Such rotation of drive gear 98 in the second direction is causedby electric motor 90 driving motor shaft 96 in a second rotarydirection.

The amount of angular movement of drive components 100 and 102 inopposite directions (i.e., the amount of relative rotation) and thecorresponding amount of axial travel of first cam plate 130 can beselected to meet the particular clutch requirements. Likewise, theprofile of cam surfaces 140A and 140B are selected to provideamplification of the clutch engagement forces. As an alternative to thearrangement shown, one of cam surfaces 140A and 142A can be non-taperedsuch that the ramping profile is configured entirely within the other ofthe cam surfaces. Also, rollers 138 are shown be cylindrical but arecontemplated to permit use of ball rollers disposed in semi-sphericalcam grooves.

In operation, when mode selector 56 indicates selection of a two-wheeldrive mode, controller 58 signals electric motor 90 to rotate motorshaft 96 in the second direction for moving first cam plate 130 until itis located in its released position, thereby releasing clutch pack 84.If mode selector 56 thereafter indicates selection of the part-timefour-wheel drive mode, electric motor 90 is signaled by controller 58 torotate driveshaft 96 in the first direction for causing lineartranslation of first cam plate 130 until it is located in its lockedposition. As noted, such movement of first cam plate 130 to its lockedposition acts to fully engage clutch pack 84, thereby coupling pinionshaft 60 to input shaft 74.

When mode selector 56 indicates selection of the on-demand four-wheeldrive mode, controller 58 energizes motor 90 to rotate driveshaft 96until first cam plate 130 is located in a ready or “stand-by” position.This position may be its released position or, in the alternative, anintermediate position. In either case, a predetermined minimum amount ofdrive torque is delivered to pinion shaft 60 through clutch pack 84 inthis stand-by condition. Thereafter, controller 58 determines when andhow much drive torque needs to be transferred to pinion shaft 60 basedon current tractive conditions and/or operating characteristics of themotor vehicle, as detected by sensors 54. As will be appreciated, anycontrol schemes known in the art can be used with the present inventionfor adaptively controlling actuation of transfer clutch 50 in adriveline application. The arrangement described for clutch actuator 52is an improvement over the prior art in that the torque amplificationprovided by geared drive unit 92 permits use of a small low-powerelectric motor and yet provides extremely quick response and precisecontrol over the position of first cam plate 130 and thus the magnitudeof the clutch engagement force applied to clutch pack 84.

To illustrate an alternative power transmission device to which thepresent invention is applicable, FIG. 5 schematically depicts afront-wheel based four-wheel drivetrain layout 10′ for a motor vehicle.In particular, engine 18 drives multi-speed transmission 20 having anintegrated front differential unit 22 for driving front wheels 24L and24R via axleshafts 26L and 26R. A power transfer unit 190 is also drivenby powertrain 16 for delivering drive torque to the input member of atorque transfer coupling 192 that is operable for selectivelytransferring drive torque to propshaft 30. Accordingly, when sensorsindicate the occurrence of a front wheel slip condition, controller 58adaptively controls actuation of torque coupling 192 such that drivetorque is delivered “on-demand” to rear driveline 14 for driving rearwheels 36L and 36R. It is contemplated that torque transfer coupling 192would include a multi-plate clutch assembly 194 and a clutch actuator196 that are generally similar in structure and function to multi-plateclutch assembly 50 and clutch actuator 52 previously described herein.

Referring now to FIG. 6, power transfer unit 190 is now schematicallyillustrated in association with an on-demand all-wheel drive systembased on a front-wheel drive vehicle similar to that shown in FIG. 5. Inparticular, an output shaft 202 of transmission 20 is shown to drive anoutput gear 204 which, in turn, drives an input gear 206 fixed to acarrier 208 associated with front differential unit 22. To provide drivetorque to front wheels 24L and 24R, front differential 22 furtherincludes a pair of side gears 210L and 210R that are connected to thefront wheels via corresponding axleshafts 26L and 26R. Differential unit22 also includes pinions 212 that are rotatably supported on pinionshafts fixed to carrier 208 and which are meshed with both side gears210L and 210R. A transfer shaft 214 is provided to transfer drive torquefrom carrier 208 to torque coupling 192.

Power transfer unit 190 includes a right-angled drive mechanism having aring gear 220 fixed for rotation with a drum 222 of clutch assembly 194and which is meshed with a pinion gear 224 fixed for rotation withpropshaft 30. As seen, a clutch hub 216 of clutch assembly 194 is drivenby transfer shaft 214 while a clutch pack 228 is disposed between hub216 and drum 222. Clutch actuator assembly 196 is operable forcontrolling engagement of clutch assembly 194. Clutch actuator assembly196 is intended to be similar to motor-driven clutch actuator assembly52 previously described in that an electric motor is supplied withelectric current for controlling relative rotation of a geared driveunit which, in turn, controls translational movement of a cam plateoperator for controlling engagement of a clutch pack 228.

In operation, drive torque is transferred from the primary (i.e., front)driveline to the secondary (i.e., rear) driveline in accordance with theparticular mode selected by the vehicle operator via mode selector 56.For example, if the on-demand four-wheel drive mode is selected,controller 58 modulates actuation of clutch actuator assembly 196 inresponse to the vehicle operating conditions detected by sensors 54 byvarying the value of the electric control signal sent to the motor. Inthis manner, the level of clutch engagement and the amount of drivetorque that is transferred through clutch pack 228 to rear driveline 14through power transfer unit 190 is adaptively controlled. Selection ofthe part-time four-wheel drive mode results in full engagement of clutchassembly 194 for rigidly coupling the front driveline to the reardriveline. In some applications, mode selector 56 may be eliminated suchthat only the on-demand four-wheel drive mode is available so as tocontinuously provide adaptive traction control without input from thevehicle operator.

FIG. 7 illustrates a modified version of FIG. 6 wherein an on-demandfour-wheel drive system is shown based on a rear-wheel drive motorvehicle that is arranged to normally deliver drive torque to reardriveline 14 while selectively transmitting drive torque to front wheels24L and 24R through torque coupling 192. In this arrangement, drivetorque is transmitted directly from transmission output shaft 202 totransfer unit 190 via a drive shaft 230 interconnecting input gear 206to ring gear 220. To provide drive torque to the front wheels, torquecoupling 192 is shown operably disposed between drive shaft 230 andtransfer shaft 214. In particular, clutch assembly 194 is arranged suchthat drum 222 is driven with ring gear 220 by drive shaft 230. As such,actuation of clutch actuator 196 functions to transfer torque from drum222 through clutch pack 228 to hub 216 which, in turn, drives carrier208 of front differential unit 22 via transfer shaft 214. Again, thevehicle could be equipped with mode selector 56 to permit selection bythe vehicle operator of either the adaptively controlled on-demandfour-wheel drive mode or the locked part-time four-wheel drive mode. Invehicles without mode selector 56, the on-demand four-wheel drive modeis the only drive mode available and provides continuous adaptivetraction control without input from the vehicle operator.

In addition to the on-demand 4WD systems shown previously, the powertransmission technology of the present invention can likewise be used infull-time 4WD systems to adaptively bias the torque distributiontransmitted by a center or “interaxle” differential unit to the frontand rear drivelines. For example, FIG. 8 schematically illustrates afull-time four-wheel drive system which is generally similar to theon-demand four-wheel drive system shown in FIG. 7 with the exceptionthat power transfer unit 190 now includes an interaxle differential unit240 that is operably installed between carrier 208 of front differentialunit 22 and transfer shaft 214. In particular, output gear 206 is fixedfor rotation with a carrier 242 of interaxle differential 240 from whichpinion gears 244 are rotatably supported. A first side gear 246 ismeshed with pinion gears 244 and is fixed for rotation with drive shaft230 so as to be drivingly interconnected to rear driveline 14 throughgearset 220 and 224. Likewise, a second side gear 248 is meshed withpinion gears 244 and is fixed for rotation with carrier 208 of frontdifferential unit 22 so as to be drivingly interconnected to the frontdriveline. Torque transfer mechanism 192 is now shown to be operablydisposed between side gears 246 and 248. As such, torque transfermechanism 192 is operably arranged between the driven outputs ofinteraxle differential 240 for providing a torque biasing and sliplimiting function. Torque transfer mechanism 192 is shown to againinclude multi-plate clutch assembly 194 and clutch actuator assembly196. Clutch assembly 194 is operably arranged between transfer shaft 214and driveshaft 230. In operation, when sensor 54 detects a vehicleoperating condition, such as excessive interaxle slip, controller 58adaptively controls activation of the electric motor associated withclutch actuator assembly 196 for controlling engagement of clutchassembly 194 and thus the torque biasing between the front and reardrivelines.

Referring now to FIG. 9, a schematic layout of a drivetrain 10A for afour-wheel drive vehicle having powertrain 16 delivering drive torque toa power transfer unit, hereinafter referred to as transfer case 290.Transfer case 290 includes a rear output shaft 302, a front output shaft304 and a torque coupling 292 therebetween. Torque coupling 292generally includes a multi-plate clutch assembly 294 and apower-operated clutch actuator 296. As seen, a rear propshaft 306couples rear output shaft 302 to rear differential 34 while a frontpropshaft 308 couples front output shaft 304 to front differential 22.Power-operated clutch actuator 294 is again schematically shown toprovide adaptive control over engagement of a clutch assembly 294incorporated into transfer case 290.

Referring now to FIG. 10, a full-time 4WD system is shown to includetransfer case 290 equipped with an interaxle differential 310 between aninput shaft 312 and output shafts 302 and 304. Differential 310 includesan input defined as a planet carrier 314, a first output defined as afirst sun gear 316, a second output defined as a second sun gear 318,and a gearset for permitting speed differentiation between first andsecond sun gears 316 and 318. The gearset includes meshed pairs of firstplanet gears 320 and second planet gears 322 which are rotatablysupported by carrier 314. First planet gears 320 are shown to mesh withfirst sun gear 316 while second planet gears 322 are meshed with secondsun gear 318. First sun gear 316 is fixed for rotation with rear outputshaft 302 so as to transmit drive torque to the rear driveline. Totransmit drive torque to the front driveline, second sun gear 318 iscoupled to a transfer assembly 324 which includes a first sprocket 326rotatably supported on rear output shaft 302, a second sprocket 328fixed to front output shaft 304, and a power chain 330.

As noted, transfer case 290 includes clutch assembly 294 and clutchactuator 296. Clutch assembly 294 has a drum 332 fixed to sprocket 326for rotation with front output shaft 304, a hub 334 fixed for rotationwith rear output shaft 302 and a multi-plate clutch pack 336therebetween. Again, clutch actuator 296 is schematically shown butintended to be substantially similar in structure and function to thatdisclosed in association with clutch actuator 52 shown in FIGS. 3 and 4.FIG. 11 is merely a modified version of transfer case 290 which isconstructed without center differential 310 to provide an on-demandfour-wheel drive system.

Referring now to FIG. 12, a drive axle assembly 400 is schematicallyshown to include a pair of torque couplings operably installed betweendriven propshaft 30 and rear axleshafts 38L and 38R. Propshaft 30 drivesa right-angle gearset including pinion 402 and ring gear 404 which, inturn, drives a transfer shaft 406. A first torque coupling 200L is showndisposed between transfer shaft 406 and left axleshaft 38L while asecond torque coupling 200R is disposed between transfer shaft 406 andright axleshaft 38R. Each of the torque couplings can be independentlycontrolled via activation of its corresponding clutch actuator assembly226L, 226R to adaptively control side-to-side torque delivery. In apreferred application, axle assembly 400 can be used in association withthe secondary driveline in four-wheel drive motor vehicles.

A number of preferred embodiments have been disclosed to provide thoseskilled in the art an understanding of the best mode currentlycontemplated for the operation and construction of the presentinvention. The invention being thus described, it will be obvious thatvarious modifications can be made without departing from the true spiritand scope of the invention, and all such modifications as would beconsidered by those skilled in the art are intended to be includedwithin the scope of the following claims.

1. A power transmission device comprising: a rotary input member adaptedto receive drive torque from a power source; a rotary output memberadapted to transmit drive torque to an output device; a torque transfermechanism for transferring drive torque from said input member to saidoutput member, said torque transfer mechanism including a frictionclutch operably disposed between said input and output members and aclutch actuator for controlling adaptive engagement of said frictionclutch, said clutch actuator including an electric motor, a drive geardriven by said electric motor, first and second face gears meshed withsaid drive gear for rotation in opposite directions, and an applymechanism for varying the magnitude of a clutch engagement force exertedon said friction clutch in response to relative rotation between saidfirst and second face gears; and a control system for actuating saidelectric motor so as to control the direction and amount of rotation ofsaid drive gear.
 2. The power transmission device of claim 1 whereinsaid apply mechanism includes an apply plate that is axially moveablebetween first and second positions in response to rotation of said firstface gear relative to said second face gear, said apply plate isoperable in its first position to exert a minimum clutch engagementforce on said friction clutch and is further operable in its secondposition to exert a maximum clutch engagement face on said frictionclutch.
 3. The power transmission device of claim 2 wherein said applyplate is fixed for rotation with said first face gear, and wherein saidapply mechanism further includes a reaction plate fixed for rotationwith said second face gear and a cam arrangement disposed between saidapply plate and said reaction plate that is operable to cause axialmovement of said apply plate between its first and second positions inresponse to relative rotation between said first and second face gears.4. The power transmission device of claim 1 wherein said first face gearincludes a first hub segment rotatably supported on said input memberand a first ring segment having first gear teeth formed on a first facesurface, wherein said second face gear includes a second hub segmentrotatably supported on said first hub segment and a second ring segmenthaving second gear teeth formed on a second face surface, and whereinsaid drive gear is meshed with said first and second gear teeth.
 5. Thepower transmission device of claim 4 wherein said drive gear is fixed toa shaft driven by said electric motor with said drive gear locatedbetween said first and second face surfaces of said first and secondface gears, respectively.
 6. The power transmission device of claim 1wherein said control system includes a controller for receiving inputsignals from a sensor and generating electric control signals based onsaid input signals which are supplied to said electric motor forcontrolling the direction and amount of rotary movement of said drivegear.
 7. The power transmission device of claim 1 wherein said firstface gear is rotatable relative to said second face gear between a firstposition and a second position in response to activation of saidelectric motor for causing corresponding axial movement of an applyplate between a retracted position and an extended position relative tosaid friction clutch, said apply plate exerting a minimum clutchengagement force on said friction clutch when located in its retractedposition and exerting a maximum clutch engagement force on said frictionclutch when located in its extended position.
 8. The power transmissiondevice of claim 7 wherein said input member provides drive torque to afirst driveline of a motor vehicle, wherein said output member iscoupled to a second driveline of the motor vehicle, and wherein saidtorque transfer mechanism is operable to transfer drive torque from saidinput member to said output member.
 9. The power transmission device ofclaim 8 defining a transfer case wherein said input member is a firstshaft driving the first driveline and said output member is a secondshaft coupled to the second driveline, wherein location of said applyplate in its retracted position releases engagement of said frictionclutch so as to define a two-wheel drive mode and location of said applyplate in its extended position fully engages said friction clutch so asto define a part-time four-wheel drive mode, and wherein said controlsystem is operable to control activation of said electric motor forvarying the position of said apply plate between its retracted andextended positions to controllably vary the drive torque transferredfrom said first shaft to said second shaft so as to define an on-demandfour-wheel drive mode.
 10. The power transmission device of claim 8defining a power take-off unit wherein said input member provides drivetorque to a first differential associated with the first driveline, andwherein said output member is coupled to a second differentialassociated with the second driveline.
 11. The power transmission deviceof claim 1 wherein said input member is a propshaft driven by adrivetrain of a motor vehicle and said output member is a pinion shaftdriving a differential associated with an axle assembly of the motorvehicle, and wherein said friction clutch is disposed between saidpropshaft and said pinion shaft such that actuation of said clutchactuator is operable to transfer drive torque from said propshaft tosaid pinion shaft.
 12. The power transmission device of claim 1 whereinsaid input member includes a first differential supplying drive torqueto a pair of first wheels in a motor vehicle and a transfer shaft drivenby said differential, wherein said output member includes a propshaftcoupled to a second differential interconnecting a pair of second wheelsin the motor vehicle, and wherein said friction clutch is disposedbetween said transfer shaft and said propshaft.
 13. The powertransmission device of claim 1 wherein said input member includes afirst shaft supplying drive torque to a second shaft which is coupled toa first differential for driving a pair of first wheels in a motorvehicle, wherein said output member is a third shaft driving a seconddifferential interconnecting a pair of second wheels of the motorvehicle, and wherein said friction clutch is operably disposed betweensaid first and third shafts.
 14. The power transmission device of claim1 further including an interaxle differential driven by said inputmember and having a first output driving a first driveline in a motorvehicle and a second output driving a second driveline in the motorvehicle, and wherein said friction clutch is operably disposed betweensaid first and second outputs of said interaxle differential.
 15. Atorque transfer mechanism for transferring drive torque from a rotaryinput member to a rotary output member, comprising: a friction clutchhaving a clutch pack operably disposed between the input and outputmembers, and an apply plate moveable between a first position whereat aminimum clutch engagement force is exerted on said clutch pack and asecond position whereat a maximum clutch engagement force is exerted onsaid clutch pack; a clutch actuator including an electric motor, a drivegear driven by said electric motor, first and second face gears meshedwith said drive gear for rotation in opposite directions in response torotation of said drive gear, and a clutch operator for moving said applyplate between its first and second positions in response to relativerotation between said first and second face gears; and a control systemfor actuating said electric motor so as to control rotary movement ofsaid first face gear relative to said second face gear between a firstposition and a second position, wherein said apply plate is located inits first position when said first face gear is in its first positionand said apply plate is located in its second position when said firstface gear is rotated to its second position.
 16. The power transmissiondevice of claim 15 wherein said apply plate is fixed for rotation withsaid first face gear, and wherein said clutch operator includes areaction plate fixed for rotation with said second face gear and a camarrangement disposed between said apply plate and said reaction platethat is operable to cause axial movement of said apply plate between itsfirst and second positions in response to relative rotation between saidfirs and second face gears.
 17. The power transmission device of claim15 wherein said first face gear includes a first hub segment rotatablysupported on the input member and a first ring segment having first gearteeth formed on a first face surface, wherein said second face gearincludes a second hub segment rotatably supported on said first hubsegment and a second ring segment having second gear teeth formed on asecond face surface, and wherein said drive gear is meshed with saidfirst and second gear teeth.
 18. The power transmission device of claim17 wherein said drive gear is fixed to a shaft driven by said electricmotor with said drive gear located between said first and second facesurfaces of said first and second face gears, respectively.
 19. Thepower transmission device of claim 15 wherein the input member providesdrive torque to a first driveline of a motor vehicle, wherein the outputmember is coupled to a second driveline of the motor vehicle, andwherein said torque transfer mechanism is operable to transfer drivetorque from the input member to the output member.
 20. The powertransmission device of claim 19 defining a transfer case wherein theinput member is a first shaft driving the first driveline and the outputmember is a second shaft coupled to the second driveline, whereinlocation of said apply plate in its first position releases engagementof said friction clutch so as to define a two-wheel drive mode andlocation of said apply plate in its second position fully engages saidfriction clutch so as to define a part-time four-wheel drive mode, andwherein said control system is operable to control activation of saidelectric motor for varying the position of said apply plate between itsfirst and second positions to controllably vary the drive torquetransferred from said first shaft to said second shaft so as to definean on-demand four-wheel drive mode.
 21. The power transmission device ofclaim 20 wherein said control system includes a controller for receivinginput signals from a sensor and generating electric control signalsbased on said input signals which are supplied to said electric motorfor controlling the direction and amount of rotary movement of saiddrive gear.
 22. The power transmission device of claim 20 defining apower take-off unit wherein the input member provides drive torque to afirst differential associated with the first driveline, and wherein theoutput member is coupled to a second differential associated with thesecond driveline.
 23. The power transmission device of claim 15 whereinthe input member is a propshaft driven by a drivetrain of a motorvehicle and the output member is a pinion shaft driving a differentialassociated with an axle assembly of the motor vehicle, and wherein saidfriction clutch is disposed between said propshaft and said pinion shaftsuch that actuation of said clutch actuator is operable to transferdrive torque from said propshaft to said pinion shaft.
 24. The powertransmission device of claim 15 wherein the input member includes afirst differential supplying drive torque to a pair of first wheels in amotor vehicle and a transfer shaft driven by said differential, whereinthe output member includes a propshaft coupled to a second differentialinterconnecting a pair of second wheels in the motor vehicle, andwherein said friction clutch is disposed between said transfer shaft andsaid propshaft.
 25. The power transmission device of claim 15 whereinthe input member includes a first shaft supplying drive torque to asecond shaft which is coupled to a first differential for driving a pairof first wheels in a motor vehicle, wherein the output member is a thirdshaft driving a second differential interconnecting a pair of secondwheels of the motor vehicle, and wherein said friction clutch isoperably disposed between said first and third shafts.
 26. The powertransmission device of claim 15 further including an interaxledifferential driven by the input member and having a first outputdriving a first driveline in a motor vehicle and a second output drivinga second driveline in the motor vehicle, and wherein said frictionclutch is operably disposed between said first and second outputs ofsaid interaxle differential.
 27. A power transmission device,comprising: a torque coupling having a friction clutch operably disposedbetween first and second rotary members; a clutch actuator for engagingsaid friction clutch and including an electric motor driving a drivegear and a geared drive unit, said geared drive unit including a firstdrive component having a first ring segment and a second drive componenthaving a second ring segment, said first ring segment having a firstface surface with first gear teeth formed thereon and said second ringsegment having a second face surface with second gear teeth formedthereon, said first gear teeth are aligned in facing relationship withsaid second gear teeth such that said drive gear is disposed betweensaid first and second ring segments so as to be in meshed engagementwith both of said first and second gear teeth; and a control system foractuating said electric motor to control the direction and amount ofrotation of said drive gear so as to vary the magnitude of a clutchengagement force applied to said friction clutch in response to relativerotation between said first and second drive components.
 28. The powertransmission device of claim 27 wherein said clutch actuator includes anapply plate that is axially moveable between first and second positionsin response to rotation of said first drive component relative to saidsecond drive component, said apply plate is operable in its firstposition to exert a minimum clutch engagement force on said frictionclutch and is further operable in its second position to exert a maximumclutch engagement face on said friction clutch.
 29. The powertransmission device of claim 28 wherein said apply plate is fixed forrotation with said first drive component, and wherein said clutchactuator further includes a reaction plate fixed for rotation with saidsecond drive component and a cam arrangement disposed between said applyplate and said reaction plate that is operable to cause axial movementof said apply plate between its first and second positions in responseto relative rotation between said first and second drive components. 30.The power transmission device of claim 27 wherein said first drivecomponent is rotatable relative to said second drive component between afirst position and a second position in response to activation of saidelectric motor for causing corresponding axial movement of an applyplate between a retracted position and an extended position relative tosaid friction clutch, said apply plate exerting a minimum clutchengagement force on said friction clutch when located in its retractedposition and exerting a maximum clutch engagement force on said frictionclutch when located in its extended position.
 31. The power transmissiondevice of claim 30 wherein said first rotary member provides drivetorque to a first driveline of a motor vehicle, wherein said secondrotary member is coupled to a second driveline of the motor vehicle, andwherein said torque coupling is operable to transfer drive torque fromsaid first rotary member to said second rotary member.
 32. The powertransmission device of claim 31 defining a transfer case wherein saidfirst rotary member is a first shaft driving the first driveline andsaid second rotary member is a second shaft coupled to the seconddriveline, wherein location of said apply plate in its retractedposition releases engagement of said friction clutch so as to define atwo-wheel drive mode and location of said apply plate in its extendedposition fully engages said friction clutch so as to define a part-timefour-wheel drive mode, and wherein said control system is operable tocontrol activation of said electric motor for varying the position ofsaid apply plate between its retracted and extended positions tocontrollably vary the drive torque transferred from said first shaft tosaid second shaft so as to define an on-demand four-wheel drive mode.33. The power transmission device of claim 31 defining a power take-offunit wherein said first rotary member provides drive torque to a firstdifferential associated with the first driveline, and wherein saidsecond rotary member is coupled to a second differential associated withthe second driveline.
 34. The power transmission device of claim 27wherein said first rotary member is a propshaft driven by a drivetrainof a motor vehicle and said second rotary member is a pinion shaftdriving a differential associated with an axle assembly of the motorvehicle, and wherein said friction clutch is disposed between saidpropshaft and said pinion shaft such that actuation of said clutchactuator is operable to transfer drive torque from said propshaft tosaid pinion shaft.
 35. The power transmission device of claim 27 whereinsaid first rotary member includes a first differential supplying drivetorque to a pair of first wheels in a motor vehicle and a transfer shaftdriven by said differential, wherein said second rotary member includesa propshaft coupled to a second differential interconnecting a pair ofsecond wheels in the motor vehicle, and wherein said friction clutch isdisposed between said transfer shaft and said propshaft.
 36. The powertransmission device of claim 27 wherein said first rotary memberincludes a first shaft supplying drive torque to a second shaft which iscoupled to a first differential for driving a pair of first wheels in amotor vehicle, wherein said second rotary member is a third shaftdriving a second differential interconnecting a pair of second wheels ofthe motor vehicle, and wherein said friction clutch is operably disposedbetween said first and third shafts.
 37. The power transmission deviceof claim 27 further including an interaxle differential driven by saidfirst rotary member and having a first output driving a first drivelinein a motor vehicle and a second output driving a second driveline in themotor vehicle, and wherein said friction clutch is operably disposedbetween said first and second outputs of said interaxle differential.